The number fraction of iron-containing particles affects OH, HO.sub.2 and H.sub.2O.sub.2 budgets in the atmospheric aqueous phase

Reactive oxygen species (ROS), such as OH, HO.sub.2 and H.sub.2 O.sub.2, affect the oxidation capacity of the atmosphere and cause adverse health effects of particulate matter. The role of transition metal ions (TMIs) in impacting the ROS concentrations and conversions in the atmospheric aqueous pha...

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Published in:Atmospheric chemistry and physics 2022-02, Vol.22 (3), p.1989
Main Authors: Khaled, Amina, Zhang, Minghui, Ervens, Barbara
Format: Article
Language:English
Subjects:
Air pollution
Analysis
Health aspects
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Summary:Reactive oxygen species (ROS), such as OH, HO.sub.2 and H.sub.2 O.sub.2, affect the oxidation capacity of the atmosphere and cause adverse health effects of particulate matter. The role of transition metal ions (TMIs) in impacting the ROS concentrations and conversions in the atmospheric aqueous phase has been recognized for a long time. Model studies usually assume that the total TMI mass as measured in bulk aerosol or cloud water samples is distributed equally across all particles or droplets. This assumption is contrary to single-particle measurements that have shown that only a small number fraction of particles contain iron and other TMIs (FN,Fe100 %), which implies that also not all cloud droplets contain TMIs. In the current study, we apply a box model with an explicit multiphase chemical mechanism to simulate ROS formation and cycling in aqueous aerosol particles and cloud droplets. Model simulations are performed for the range of 1 % [less than or equal to] F.sub.N,Fe [less than or equal to] 100 % for constant pH values of 3, 4.5 and 6 and constant total iron mass concentration (10 or 50 ng per cubic meter of air). Model results are compared for two sets of simulations with FN,Fe100 % (FeN100) and 100 % (FeBulk). We find the largest differences between model results in OH and HO.sub.2 / O2- concentrations at pH = 6. Under these conditions, HO.sub.2 is subsaturated in the aqueous phase because of its high effective Henry's law constant and the fast chemical loss reactions of the O2- radical anion. As the main reduction process of Fe(III) is its reaction with HO.sub.2 / O2-, we show that the HO.sub.2 subsaturation leads to Fe(II) / Fe(total) ratios for FN,Fe100 % that are lower by a factor of [less than or equal to] 2 as compared to bulk model approaches. This trend is largely independent of the total iron concentration, as both chemical source and sink rates of HO.sub.2 / O2- scale with the iron concentration. We compare model-derived reactive uptake parameters γ.sub.OH and γHO2 for the full range of F.sub.N,Fe . While γ.sub.OH is not affected by the iron distribution, the calculated γHO2 values range from 0.0004 to 0.03 for F.sub.N,Fe = 1 % and 100 %, respectively. Implications of these findings are discussed for the application of lab-derived γHO2 in models to present reactive HO.sub.2 uptake on aerosols. We conclude that the iron distribution (F.sub.N,Fe) should be taken into account to estimate the ROS concentrations and oxidation potentia
ISSN:1680-7316
1680-7324